In the auto industry, demand has been rising for steel sheet provided with the properties of both shapeability and high strength so as to achieve both lighter weight of the chassis to deal with environmental problems and safety in collisions.
To deal with these needs, Japanese Unexamined Patent Publication (Kokai) No. 5-59429 discloses steel sheet utilize the transformation-induced plasticity exhibiting a high ductility by the transformation of the residual austenite in the steel sheet structure to martensite at the time of shaping. This type of steel sheet for example forms a complex structure by the addition of for example C in 0.05 to 0.4 wt %, Si in 0.2 to 3.0 wt %, and Mn in 0.1 to 2.5 wt % in the steel and controlling the temperature pattern in the process of annealing in the two-phase region, then cooling and is characterized in that the desired properties can be brought out without the use of expensive alloy elements.
When zinc plating this steel sheet by a continuous molten zinc plating system, usually the surface of the steel sheet is degreased, the surface is cleaned, then, for the purpose of forming the above-mentioned structure, the sheet is heated in an nonoxidizing furnace to form an iron oxide layer of a thickness of 50 nm to 1 μm or so on the surface of the steel sheet, annealing the sheet in a reducing furnace to reduce the iron oxide layer, then dipping the sheet in a molten zinc plating bath to plate it with zinc.
Steel sheet, however, contains large amounts of easily oxidizing elements such as Si and Mn compared with the ordinary deep drawn cold-rolled steel sheet etc., so there is the problem that the surface of the steel sheet is easily formed with Si oxides, Mn oxides, or Si and Mn complex oxides in the heat treatment performed in the above series of steps. However, in industrial scale systems, it is difficult to reduce the oxygen potential of the atmosphere in the heating step to an extent where Si or Mn will not be oxidized, so formation of Si and Mn oxides at the surface of the steel sheet is substantially unavoidable. Further, if the surface of the steel sheet is formed with an Si oxide layer or Mn oxide layer, there are the problems that in the process of production of the molten zinc plated steel sheet, the wettability between the surface of the steel sheet and the molten plating remarkably deteriorates so the plating will not be deposited at parts and the surface of the steel surface will be exposed, that is, the phenomenon of “plating gaps” will arise, and the bondability of the plating will deteriorate. In particular, plating gaps are normally on the millimeter order in size, so its presence can be seen.
To deal with this problem, Japanese Unexamined Patent Publication (Kokai) No. 55-122865 discloses the method of forming a 40 to 1000 nm iron oxide layer on the surface of a steel sheet in a heat treatment step by a nonoxidizing furnace in a continuous molten zinc plating step so as to prevent outward diffusion of the Si or Mn in the reduction step, suppress the formation of the Si oxide layer, and improve the plating properties. With this method, however, if the reduction time is too long for the thickness of the iron oxide layer, Si will become dense at the surface of the steel sheet and an Si oxide layer will be formed, while if the reduction time is too short, iron oxide will remain on the surface of the steel sheet and the wettability will not be improved. Further, in recent continuous molten zinc plating systems, annealing systems using radiant type heating furnaces rather than nonoxidizing furnaces are becoming the mainstream. In such systems, there was the problem that the above method could not be used.
Japanese Unexamined Patent Publication (Kokai) No. 2-38549 proposes a method of pre-plating the surface of the steel sheet before annealing with the purpose of suppression outward diffusion of Si or Mn. However, with the pre-plating method, a plating system is required, so this cannot be employed when there is no space. Further, with steel sheet containing a large amount of Si or Mn, there was the problem that an increase in the amount of pre-plating is required and a drop in the productivity is invited.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2000-309824 discloses as a method for preventing selective oxidation of the Si or Mn at the time of annealing the method of hot rolling the steel sheet, then heat treating it in the state with the black skin scale still attached in an atmosphere where reduction will substantially not occur and in a temperature range of 650 to 950° C. so as to form a sufficient internal oxide layer in the base iron surface layer. With this method, however, in addition to the conventional continuous molten zinc plating step, a heat treatment step for forming the internal oxide layer and a pickling treatment step become necessary, so there was the problem that a rise in production costs was invited.